This invention relates to a method for the manufacture of concrete products by the thermal curing of concrete mix confined within airtightly closed molding frames.
It is widely known that for the thermal curing of concrete mix, there is generally adopted a method which comprises treating the concrete mix with steam under atmospheric pressure. According to this conventional method, since the concrete mix is heated with steam under atmospheric pressure, the highest temperature to which the concrete mix can be heated is 100.degree. C.
By this method, the cement in the concrete mix reacts with water to produce a low-crystallinity tobermorite of the structural formula 3CaO.2SiO.sub.2.3H.sub.2 O as shown by the following reaction formula: ##STR1##
As is clear from this reaction formula, in concrete products cured at temperatures not exceeding 100.degree. C., free calcium hydroxide is present besides the aforementioned low-crystallinity tobermorite.
For the purpose of improving the quality of concrete products, an autoclave method is available which cures the concrete mix at elevated temperatures under increased pressure.
This autoclave method comprises causing pre-cured concrete products released from molds to be cured at elevated temperatures in the range of from 150.degree. to 200.degree. C. under increased pressure in the range of from 5 to 16 kg/cm.sup.2. Consequently, the part of the silica which has escaped reaction during the pre-curing performed at temperatures not exceeding 100.degree. C. is allowed to react with the aforementioned free calcium hydroxide to produce a large amount of a stable, crystalline hydride (5CaO.6SiO.sub.2.5H.sub.2). Owing to this curing reaction, the concrete products cured by the autoclave method acquire sufficient strength in a shorter time than the concrete products cured at temperatures not exceeding 100.degree. C. Thus, the autoclave method is notably effective in improving the quality of concrete products. The autoclave method nevertheless has the following drawbacks:
(1) Since the curing of concrete products takes place under harsh temperature and pressure conditions of 200.degree. C. and 16 kg/cm.sup.2, there must be installed an autoclave capable of withstanding these conditions. Thus, the autoclave method entails a remarkably high equipment cost.
(2) Since the autoclave interior is exposed to elevated temperatures and increased pressure, its inner volume is limited by reason of operational safety. Thus, the autoclave method fails to provide the desired curing for concrete products of large dimensions.
(3) Since fresh concrete mix is pre-cured (generally by means of steam) until it acquires strength enough to withstand possible impacts exerted thereon during the work of mold release and, after this strength is acquired, the pre-cured concrete products are released from their molds and then are subjected to the final curing in the autoclave, the total time required for curing is too long for the curing of concrete products to be efficient. Besides, the operational efficiency is inferior because the operation inevitably involves an extra task of the transfer of pre-cured concrete products released from the molds into the autoclave. Moreover, the concrete products suffer from inferior dimensional accuracy because of the phenomenon of thermal expansion.
An object of the present invention is to provide a method which permits the easy manufacture of concrete products possessing properties far excelling those of concrete products manufactured by the autoclave method.
Another object of this invention is to provide a method which permits the easy manufacture of concrete products of high dimensional accuracy and good quality with a notably shorter curing time.